In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Recently, the solid golf balls, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. In the solid golf ball, many techniques for further improving the performance thereof, such as technique for improving the rebound characteristics, technique for varying hardness distribution, have been suggested, and the solid golf balls having good shot feel while maintaining excellent flight performance have been provided (Japanese Patent Kokai Publication Nos. 149502/2001, 293996/2002, 355338/2002, 33447/2003 and the like).
In Japanese Patent Kokai Publication No. 149502/2001, a golf ball comprising vulcanized molded article of rubber composition as a structural element is disclosed, the rubber composition mainly comprises a base rubber containing 10 to 100 mass % of polybutadiene containing a cis-1,4 bond of not less than 90%, and the polybutadiene has a 5 mass % toluene solution viscosity at 25° C., η (mPa·s) of not less than 200 and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 2.0 to 4.0.
In Japanese Patent Kokai Publication No. 293996/2002, a rubber composition containing (a) 50 parts to 100 parts by weight of modified polybutadiene rubber having a 1,4-cis bond content of 80 mol % to 100 mol % and a 1,2-vinyl bond content of 0 mol % to 2 mol %, which is modified with an alkoxysilyl group-containing compound, (b) 50 part to 0 parts by weight of diene rubber other than the above-mentioned component (a) (wherein the total amount of component (a) and component (b) is 100 parts by weight), (c) 10 parts to 50 parts by weight of a crosslinkable monomer, (d) 5 parts to 80 parts by weight of an inorganic filler, and (e) 0.1 part to 10 parts by weight of an organic peroxide, is disclosed.
In Japanese Patent Kokai Publication No. 355338/2002, a golf ball comprising a hot-molded product of a rubber composition comprising,    based on 100 parts by weight of a base rubber composed of (a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of at least 60% and a 1,2-vinyl content of at most 2%, having a 5 mass % toluene solution viscosity η at 25° C. of not less than 600 mPa·s, and satisfying the following relationship:10×B+5≦A≦10×B+60wherein A is Mooney viscosity (ML1+4(100° C.)) of the polybutadiene and B is a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of the polybutadiene, in combination with (b) 0 to 80 wt % of a diene rubber other than the component (a),    (c) 10 to 60 parts by weight of an unsaturated carboxylic acid and a metal salt thereof or both,    (d) 0.1 to 5 parts by weight of an organosulfur compound,    (e) 5 to 80 parts by weight of an inorganic filler, and    (f) 0.1 to 5 parts by weight of an organic peroxide:wherein the hot-molded product has a difference in JISC hardness between the center and surface thereof of more than 15 and up to 40 JIS-C hardness units, is disclosed.
In Japanese Patent Kokai Publication No. 333447/2003, a solid golf ball comprising a core containing at least one layer, and a cover containing at least one layer and covering the core, wherein the at least one layer of the core is molded by vulcanizing a rubber composition including: (a) a base rubber, (b) a co-crosslinking agent, (c) an organic peroxide, and (d) a polysulfide compound represented by the following formula:
(where at least one of R1 to R5 and at least one of R6 to R10 are each Br, and n is an integer of not less than 2), is disclosed.
However, in the resulting golf balls described above, sufficient performances have not been accomplished, and therefore golf balls having further improved performance have been required. In addition, it has been problem in the golf balls that the durability and rebound characteristics are degraded by softening the core such that the difference between the surface hardness and center hardness is increased because of soft and good shot feel.